1. Field of the Invention
The present invention relates to a multi-value optical transmitter for converting an electrical signal into an optical signal and transmitting the optical signal.
2. Description of the Related Art
In order to realize a large capacity of a wavelength division multiplex (WDM) optical communication system, it is useful to increase a transmission rate per wavelength. When a symbol rate of symbols transmitted to an optical transmission line is increased without changing a modulation method, there is a problem in that wavelength dispersion tolerance of the optical transmission line reduces because an allowable residual dispersion amount is inversely proportional to the square of the symbol rate. It is necessary to execute electrical signal processing at high speed, and hence there is a problem in that the cost of an analog electrical part increases.
Therefore, in recent years, researches for improving signal multiplicity per symbol without increasing the symbol rate so as to realize the large capacity of the system have been actively conducted.
Known examples of a method of improving the signal multiplicity include multi-value modulation methods such as a quadrature phase shift keying (QPSK) method of assigning a binary value (multiplicity is two) to each symbol to increase a transmission capacity two times, a 16-quadrature amplitude modulation (16QAM) method of assigning a quaternary value (multiplicity is four) to each symbol to increase the transmission capacity four times, and a 16-amplitude phase shift keying (16APSK) method.
In general, when any of the multi-value modulation methods is executed, an I/Q modulator is used as an optical modulator. The I/Q modulator is an optical modulator capable of independently generating orthogonal optical electric field components (I channel and Q channel) and has a special structure in which Mach-Zehnder (MZ) modulators are connected in parallel.
For example, when the QPSK method is to be executed, a dual parallel MZ modulator (DPMZM) is used in which two MZ modulators are connected in parallel (see, for example, JP 2004-516743 A).
When the 16QAM method is executed, the DPMZM or a quad parallel MZ modulator (QPMZM) in which two DPMZMs are connected in parallel is used (see, for example, T. Sakamoto, et al., “50-Gb/s 16 QAM by a quad-parallel Mach-Zehnder modulator”, ECOC2007, PD2.8, 2007).
Even when any of the DPMZM and the QPMZM as described above is used, the number of MZ modulators increases, and hence there is a problem that a manufacturing cost and the number of bias control points increase.
Therefore, it is expected to use a dual-electrode MZ modulator (dual drive MZM (DDMZM)) in which two phase modulators are connected in parallel, so as to realize the multi-value modulation (see, for example, K-P. Ho, et al., “Generation of Arbitrary Quadrature Signals Using One Dual-Drive Modulator”, IEEE J. Lightwave Technol., Vol. 23, No. 2, February 2005, pp. 764-770, and D. J. Krause, et al., “Demonstration of 20-Gb/s DQPSK with a Single Dual-Drive Mach-Zehnder Modulator”, IEEE Photon. Technol. Lett., Vol. 20, No. 16, Aug. 15, 2008, pp. 1363-1365).
The dual-electrode MZ modulator is an optical part widely applied as a push-pull optical modulator to a normal optical transmitter-receiver, and hence a reduction in cost may be realized. In addition, a light insertion loss may be reduced because of the structure in which light passes through the MZ modulator only once.
Next, a case where the QPSK is executed using the dual-electrode MZ modulator is considered with reference to FIGS. 5A and 5B. With respect to the QPSK using the dual-electrode MZ modulator, a method with a signal point configuration illustrated in FIG. 5A (three-value driving) has been demonstrated up to now (see, for example, D. J. Krause, et al., “Demonstration of 20-Gb/s DQPSK with a Single Dual-Drive Mach-Zehnder Modulator”, IEEE Photon. Technol. Lett., Vol. 20, No. 16, Aug. 15, 2008, pp. 1363-1365).
FIG. 5A illustrates a locus of change of an optical electric field on a complex plane in a case where the dual-electrode MZ modulator is driven with double over sampling. In FIG. 5A, black circles correspond to QPSK signal points.
In order to take advantage of a low insertion loss which is a feature of the dual-electrode MZ modulator, modulator driving based on an four-value electrical signal, which is superior to the three-value driving, is used. When the four-value driving is performed, a loss in the dual-electrode MZ modulator is smaller than a loss in the case of the three-value driving by approximately 3 dB (see, for example, K-P. Ho, et al., “Generation of Arbitrary Quadrature Signals Using One Dual-Drive Modulator”, IEEE J. Lightwave Technol., Vol. 23, No. 2, February 2005, pp. 764-770).
FIG. 5B illustrates a locus of change of an optical electric field on a complex plane in a case where the QPSK of the four-value driving is executed using the dual-electrode MZ modulator.
As in a case where normal on-off keying (OOK) transmission is executed, even when the multi-value modulation is executed using the dual-electrode MZ modulator as an optical modulator, in order to obtain stable optical transmission signal quality, it is essential to stabilize a direct current (DC) bias for determining an operating point of the dual-electrode MZ modulator.
However, the conventional technologies have the following problem.
As described above, when the dual-electrode MZ modulator is used as the optical modulator, it is essential to stabilize the DC bias for determining the operating point of the dual-electrode MZ modulator. However, up to now, there is a problem that DC bias stabilization control technologies for the QPSK of four or more values using the dual-electrode MZ modulator are not disclosed.